Embodiments of the innovation pertain generally to the field of electrical power conversion and particularly to DC to AC inverters utilizing semiconductor switches.
Electrical power inverters typically utilize pairs of semiconductor switches that are connected together across a DC bus or supply lines to which a DC voltage source is connected. The switches are alternately turned on and off in a selected switching sequence to provide AC power to a load connected to a node between the two switches. The high side semiconductor switches are almost always selected to be n-type devices because of their superior switching characteristics and low on-resistance compared to p-type devices. As a result, the high side switch requires a floating voltage source and level-shift function that contributes to the cost and complexity of the inverter gate drive. A single pair of semiconductor switches connected in this manner may be used by itself to provide single phase AC power to a load, or two pairs of switches may be connected together in a conventional H-bridge configuration, for single phase power, three pairs of switches for three phase power, etc. Each pair of switches may be considered a phase leg of a single phase or multiphase inverter.
Dead time is almost always added to the gate drive signals provided to the two switches of a phase leg to ensure that one of the switches is completely turned off before the other switch is turned on. Otherwise, if both of the switches were turned on simultaneously, a short circuit current through the switches could burn out the switches or damage other circuit components because the two switches are connected in series across the DC bus lines. This condition is sometimes called “shoot through.” However, the presence of dead time can add a significant amount of undesired non-linearity and harmonic distortion to output voltage waveforms. The output waveform distortion and voltage amplitude loss of the fundamental-frequency components becomes worse as either the fundamental frequency or the carrier frequency increases.
Different methods for compensating for dead time are known, including sensing current flow through the switches and ensuring the turn-off of a conducting switch before the other is turned on. See U.S. Pat. Nos. 4,126,819, 5,646,837 and 5,859,519 and published U.S. patent application US2001/0048278A1. Such circuits require significant additional components, with significant added cost, or still require delays between turn-off and turn-on of the switches with corresponding dead time. U.S. Pat. No. 6,909,620 has an output node between the two switches, with a series diode or connector switch between the output node and the low side switch, and the junction between the diode or connector switch and the low side switch electrically connected directly to the gate of the high side switch. If the low side switch is still conducting at the time that the high side switch receives a command to turn on, the gate of the high side switch will be biased so that the switch is held off until current stops flowing through the low side switch and, conversely, if the high side switch is still on at the time that the low side switch is turned on, the gate of the high side switch will be biased to insure its immediate turn-off, thereby preventing a condition under which the high side and low side switches are turned on at the same time.
Yet as the fast switching of the high speed switches occurs, the drain-gate capacitance creates a path for parasitic current to flow into the internal gate resistance, which causes a voltage spike on the gate, risking unwanted turn on, and a shoot through condition to possibly occur.